I ready somewhere that ~ 70% of the trades in the contemporary stock market are done using algorithmic trading. It's hard to say what the exact percentage is, but with the increase in volatility, something out of historic norms is occurring. There are many different models for trading in stocks and the assumption of this writer is that they all have been programmed into some kind of computer and they are activity being used to trade in this market. Often times the human assumptions used to program these machines are in conflict with each other, so it's not a surprise that the stock market tends to jump around much more than usual.